Due to environmental considerations, plastic bottle processors are considering alternatives to polyvinyl chloride (PVC) for extrusion blow molding applications. The switch from PVC is being driven by incineration concerns and the lack of a major recycling network. Poly(ethylene terephthalate) [PET]is currently being developed for extrusion blow molding processes. The extrusion blow molding process contains the following steps:
(1) Melting the resin in an extruder
(2) Extrusion of pipe from a die
(3) Forming and trimming the pipe into a preform
(4) Inflation of the preform into a container
(5) Ejection of the container
Development of PET for extrusion blow molding applications has been plagued by the need to obtain a polymer with sufficient melt strength to prevent parison sag during bottle formation. In order to obtain a polymer with adequate melt strength, resin suppliers have begun to develop high molecular weight linear polyesters and high molecular weight branched polymers. The high molecular weight resins have very high melt viscosities (60,000-200,000 poise at 265.degree. C.). Processing the viscous polymer into a homogeneous melt that can be blown into bottlers at production rates is a challenge for the extrusion blow molding industry. Typically, slower processing rates or special mixing screws are required to facilitate the extrusion of high melt viscosity polymers without rheological inhomogeneities. The rheological inhomogeneities appear optically as "waves" and/or agglomerates in the bottle sidewalls.
Solid state polycondensation is typically used in the industry to obtain high molecular weight polyesters. The polyesters are normally in the form of a three dimensional figure, such as generally spherical, cubical, cylindrical, etc. During solid state polymerization, the core or portions remote from the surface of a pellet undergo a small change in molecular weight, while the outside surface of the pellet undergoes a significant increase in molecular weight. The pellet surface molecular weight is particularly increased if the polyester is branched with a polyfunctional branching agent. Processing of branched polyesters with large core to surface molecular weight gradients results in containers with many rheological inhomogeneities.
There is considerable literature, such as patents and publications disclosing branched polyesters with polyfunctional branching agents. Such patents include U.S. Pat. Nos. 2,895,946, 3,502,620, 3,406,045, 3,576,773, 3,580,874, 4,217,440 and British 1,027,613. There are patents detailing branched polyesters and copolyesters for processing into bottles. U.S. Defensive Publication T954,005 discloses the use of branching agents to modify polyesters and copolyesters in the melt polymerization to give a polymer with sufficient melt strength for extrusion blow molding applications, but it does not disclose diethylene glycol or the use of solid state polymerization to obtain high molecular weight polymers. U.S. Pat. No. 4,983,711 discloses a class of amorphous copolyesters suitable for extrusion blow molding applications; however, it does not address the use of diethylene glycol, precursor inherent viscosity (I.V.) or the use of solid state polymerization to obtain high molecular weight polyesters suitable for extrusion blow molding applications.